The environmental and other governmental measures adopted by several countries against the use of tetraethyllead in motor-fuel led the petroleum industry to seek other additives, including oxygen-containing additives, for improving the octane number of motor-fuel. Among these additives, asymmetric ethers, and more particularly methyl tert-butyl ether (MTBE), have proved to be very efficient gasoline additives. The most common method for the preparation of MTBE comprises the reaction of isobutene with methanol.
Isobutene is also used as starting material for the production of other valuable compounds, such as t-butyl alcohol (used as solvent), t-butyl phenol (used as stabilizer), low molecular weight polymers (used to improve the viscosity index of lubricating oils), etc. As a result of this increased interest in isobutenes, the present availability of isobutene does not allow the production of sufficient amounts of these derivatives to satisfy their potential market.
Accordingly, it can be seen that presently there is a need for a process to simply and economically produce isobutene, and particularly, a process that can utilize starting materials which are readily available.
One of the continuing problems in a petroleum refinery when using catalytic cracking processes is handling the very large amounts of gas produced. Catalytic cracking, and especially fluid catalytic cracking (FCC), is widely used in petroleum refineries. Refiners have more capacity for catalytic cracking than for any other single process except distillation. Since catalytic cracking is a non-hydrogenative process, it can be appreciated that huge amounts of olefinic gases are produced. Whenever the severity of a catalytic cracker is increased or the throughput is increased, even more olefinic gases are produced.
Recovering these enormous amounts of gas for further reaction requires large capital outlays for compressors and gas handling equipment. The alternative is to burn the olefinic gases as fuel for other parts of the refinery or as waste. Unfortunately, because the quantities of gas are huge and the capital costs are high, these gases are too often burned instead of recovered and reacted. C.sub.3 and lower gases comprise the majority of wasted gases.
It can be appreciated that there is a highly intensive search for efficient, economical processes which would allow these reactive olefinic gases to be used further as chemicals rather than to be wasted. These olefinic gases often contain propylene, which is of particular interest in the present invention.
Several processes for upgrading these olefinic gases have been described in the literature. U.S. Pat. No. 4,414,423 to S. J. Miller discloses a method for upgrading feeds containing normally gaseous olefins by an oligomerisation reaction in at least two steps leading to the formation of high boiling point hydrocarbons. This method may be applied to propylene or to a mixture of propane and propylene as starting materials. Normally, liquid olefins are formed by oligomerisation in a first step, and are subsequently converted into higher oligomers in a second step. The object of the method described in this patent is to give good yields of high boiling point hydrocarbons. During the first step, a minimal amount of C.sub.4 olefins (of which isobutene is only one of the three isomers), is formed starting from a mixture of propane and propylene. Therefore, such a method cannot be used for the selective production of isobutene from propylene or from a gaseous feed which contains propylene. U.S. Pat. No. 4,417,086 to Miller and U.S. Pat. No. 4,417,088 to Miller disclose processes for oligomerizing liquid olefins using intermediate pore size molecular sieves. However, none of these presently known processes can be utilized for the production of significant amounts if isobutene.